Examples of properties that a capacitor oil is mainly required to have include high breakdown voltage, high hydrogen gas absorbability, low viscosity and low melting point. In recent years, electrical insulating oils with a high breakdown voltage are being used worldwide. Unlike the past years, an electrical insulating oil having such excellent low temperature properties that make it possible to be used in extremely low temperature districts where such an oil has never been used before has been demanded, accompanied with economic growth. It is known that if solids are produced in an electrical insulating oil during the use thereof, discharge is likely to occur from the solidified portions. Therefore, an electrical insulating oil from which some components are likely to precipitate as solids under this environment cannot be used. Whilst, as the temperature at which an electrical insulating oil may be used depends on the temperature of the environment of usage thereof, the oil needs to have not only properties at extremely-low temperatures but also properties in the vicinity of 20 to 30° C.
Over a long period of time, a mixture of benzyltoluene and dibenzyltoluene has been used as an electrical insulating oil with a high breakdown voltage. Although benzyltoluene is high in aromatic carbon ratio per molecule, high in hydrogen gas absorbability and excellent in withstand voltage characteristics, according to some literatures, the melting points of 3 types of positional isomer of benzyltoluene, i.e., o-isomer, m-isomer and p-isomer are +6.6° C., −27.8° C. and +4.6° C., respectively, and thus cannot be deemed low.
In order to solve such problems, Japanese Patent Application Laid-Open Publication No. 60-87231 (Patent Literature 1) has proposed to mix benzyltoluene produced by reacting toluene and benzyl chloride with a ferric chloride catalyst, with dibenzyltoluene that is a coproduct. ARKEMA has commercialized an electrical insulating oil composition under the name of “JARYLEC C-101” which is the same in technical sense as the proposal of Patent Literature 1. Patent Literature 1 discloses an oligomer mixture of triarylmethane, which is, however, substantially a mixture of benzyltoluene and dibenzyltoluene. Patent Literature 1 describes at page 3 “monobenzyltoluene has a defect that it crystallizes at −20° C. after supercooling” and therefore, the composition is produced by mixing dibenzyltoluene to restrain the crystallization.
However, addition of compounds such as dibenzyltoluene is not a good measure for the following three reasons. That is, even though the freezing point depression could be expected by addition of dibenzyltoluene, it is not decreased as much as the mass of the addition of dibenzyltoluene due to the high molecular weight thereof. The freezing point depression occurs proportionally to the mol concentration of the material to be added, but with the amount of dibenzyltoluene in the order of 20 percent by mass as added in the above-described product JARYLEC C-101, the crystallization temperature can be decreased only by 6 to 8° C. when calculated from the mol concentration.
Secondly, dibenzyltoluene merely increases the viscosity of an insulating oil and thus decreases the mobility of the solution molecules thereby apparently restrains the oil from precipitating. Therefore, the precipitation of the oil as crystals can be found if carefully cooling the oil.
The third reason is that dibenzyltoluene has high biological accumulation properties. In recent years, Stockholm Convention or the like has started to impose an international restriction on substances having a high toxicity. Although no such a restriction has been imposed on dibenzyltoluene itself, it has been designated as Type I Monitoring Chemical Substance in Japan due to its high biological accumulation properties. The use of this substance is allowed in the form of essential use where the purposes of use are restricted, but from now, tightening of regulations on the high toxicity substances is inevitable, and thus alternative materials with a low toxicity have been demanded.
Japanese Patent Application Laid-Open Publication No. 62-180907 (Patent Literature 2) discloses an electrical insulating oil composed of benzyltoluene and describes about the relationship between the ratio of each isomer in the benzyltoluene and the properties of the insulating oil. The melting point of an isomer mixture is −43° C. or higher, and thus the insulating oil is not sufficient in low temperature properties.
Japanese Patent Application Laid-Open Publication No. 63-64217 (Patent Literature 3) describes an electrical insulating oil comprising benzyltoluene and ditolylmethane, and from the description, it is appreciated that the properties of the oil is significantly varied on the type of substances to be blended and the blend ratio thereof. That is, an electrical insulating oil infrequently brings out properties as unexpected by theory depending on substances to be blended.
Onthe other hand, 1-phenyl-1-xylylethane or 1-phenyl-1-ethylphenylethane is easily produced and has excellent properties such as relatively high breakdown voltage and small dielectric loss and thus have been widely used. For example, a composition comprising 1-phenyl-1-(2,4-dimethylphenyl)ethane or 1-phenyl-1-(2,5-dimethylphenyl)ethane has been proposed as an electrical insulating oil composition which is excellent in breakdown voltage and dielectric loss and also particularly excellent in oxidation stability (Japanese Patent Application Laid-Open Publication No. 57-50708: Patent Literature 4).
However, an electrical insulating oil composition comprising 1-phenyl-1-xylylethane or 1-phenyl-1-ethylphenylethane has a pour point of −47.5° C. or lower and a very low melting point but has a problem that it is not sufficient in insulation properties for a capacitor in particular in a low temperature range of 0° C. or lower because its 40° C. viscosity is in the order of 5.0 mm2/s, which is high.
Meanwhile, 1,1-diphenylethane is high in breakdown voltage and hydrogen gas absorbability and has a 40° C. viscosity of 2.8 mm2/s and a freezing point of −18° C., which is low and thus is a potential substance for an electrical insulating oil with an excellent low temperature properties. Although 1,1-diphenylethane is low in freezing point, it cannot be used alone in a temperature range which is lower than the freezing point.